1. Field of the Invention
The invention relates to a light beam scanning apparatus to scan a scanning object by a light beam in order to record or read an image or the like and, more particularly, to a light beam detecting mechanism in which when a predetermined position serving as a positioning reference point is scanned by a light beam, such a scan is detected and a predetermined signal is generated.
2. Related Background Art
FIG. 1 is a schematic diagram showing the construction of a conventional example of a laser printer in which a light beam scanning apparatus is assembled.
The light scan in the laser printer which is used to record an image or the like is executed in the following manner.
A laser beam which is emitted from a semiconductor laser 701 passes through a collimating lens 702, a cylindrical lens 703, and the like and enters a rotational polygon mirror 704 which is rotated at a predetermined speed. The laser beam is reflected by each surface of the polygon mirror 704, thereby deflecting and scanning within a predetermined angle. After that, the reflected light passes through the optical system such as f.theta. lens group 705, light path folding mirror 708, and the like which are provided to form the beam and to scan at a uniform speed. The light subsequently optically scans a photo sensitive material 709 as a recording medium.
The light scan, printing operation, and the like as mentioned above are executed by a control unit (not shown).
The control unit drives the semiconductor laser 701 by an on/off signal based on an image signal which is supplied from the outside. The laser beam is modulated in accordance with the image signal. The drum-shaped photo sensitive material 709 is rotated in the direction perpendicular to the main scanning direction every main scan. The optical recording operation of an image, characters, or the like is finished by the actuation of the semiconductor laser 701 and the rotation of the photo sensitive material 709.
In order to prevent adverse influence on the image to be formed, the positional precision of the laser scan relative to the image signal needs to be held to an extreme high precision to avoid a variation in the position precision of each main scan. For this purpose, in such a kind of laser printer or the like, there is provided a light beam detecting mechanism having a light beam receiving section 707 for receiving the light folded by a submirror 706 as shown in the diagram and for detecting that the laser beam which had been deflected and scanned by the rotational polygon mirror 704 has reached a predetermined position. The light beam detecting mechanism generates a sync signal to decide the recording starting position on the photo sensitive material 709 on the basis of a detection signal of the light beam receiving section 707. The control unit allows the image signal to be generated in accordance with the timing of the sync signal, thereby always keeping the same writing position.
The laser beam of a predetermined intensity is not always irradiated to the light beam receiving section 707 due to a surface accuracy or a surface inclination of each surface of the rotary polygon mirror, dust, or dirt of the rotary polygon mirror or the optical system, or the like. Therefore, if the apparatus is constructed so as to generate the sync signal when the level of output of the light receiving section exceeds a predetermined threshold value, a variation of the light amount occurs every main scan. If the level of output which is received by the light beam detecting mechanism fluctuates, the generation timing of the sync signal is deviated.
To prevent the above inconveniences, there has been proposed a method whereby two light receiving sections (the first light receiving section and the second light receiving section) are closely arranged on the main scan line, a ratio of output signals of the first and second light receiving sections is obtained by a calculating circuit, and the sync signal is generated when the output signal ratio is equal to a predetermined value.
On the other hand, as for the semiconductor laser, since the output characteristics are easily influenced by a temperature change and a fluctuation of the light amount is large, a method whereby the semiconductor laser device is devised so as to keep a predetermined temperature by a Peltier device or the like and the output fluctuation is suppressed has been proposed.
In the above conventional light beam detecting mechanisms, in case of the light beam detecting mechanism having only one light receiving section, there is a problem such that the generation timing of the sync signal is deviated due to the output fluctuation of the light beam.
In case of the light beam detecting mechanism having two light receiving sections, since the sync signal is obtained by setting off the output fluctuation of the light beams which are irradiated to the light receiving sections, the above problem does not occur. However, there is a problem such that an out-of focus state of the light beam, out-of main scanning position, or the like due to the precision of the whole apparatus by a temperature change, an aging change, or the like cannot be detected.
In case of suppressing the output fluctuation of the semiconductor laser device so as to keep a predetermined temperature by a Peltier device or the like, the generation timing of the sync signal is not deviated. However, since the output characteristics of the semiconductor laser device are also changed by a change with the passage of time, there is a case where a wavelength fluctuation occurs and the laser beam of a wavelength which is different from the initial set wavelength is generated. When the wavelength fluctuates, an out-of focus state generally occurs in the laser scan optical system or the like in which the chromatic aberration is not sufficiently corrected. Any one of the above light beam detecting mechanisms cannot cope with such an out-of focus state.
In many cases, the user is aware of such various inconveniences as mentioned above until after completion of recording, so that both time and costs are wasted.